One possible plot could be that Harry discovers a young Tom in a difficult situation, perhaps an orphanage similar to his own past. He decides to take Tom under his wing out of a sense of empathy. Harry then tries to guide Tom away from the dark path he originally took in the canon. For example, he might introduce Tom to the world of positive magic and friendship early on, which could change Tom's perspective on life and magic.
In a 'harry adopts tom fanfic', maybe Harry travels back in time and meets a young Tom. Realizing the potential for good in Tom, he adopts him. Harry would then use his knowledge of Tom's future actions to prevent the bad things from happening. He could start by teaching Tom about love and acceptance, which Voldemort never truly understood. This could lead to a completely different future for the wizarding world.
One possibility is that Harry, having seen the dark path Tom Riddle took, decides to take him in early. He might teach Tom about love and friendship, things he lacked growing up. Harry could introduce Tom to the wizarding world in a more positive light, showing him the joys of Hogwarts and the value of good relationships, thus preventing him from turning into Voldemort.
In a 'Grindelwald Adopts Harry' fanfic, Grindelwald could be seeking to create an heir of sorts. He adopts Harry when he is very young. Grindelwald might expose Harry to a world of magic that is different from what he would have known at Hogwarts. Harry could be taught magic that is more about power and control. However, as Harry gets older, he might discover the truth about Grindelwald's dark deeds and his plans for the wizarding world. This could lead to a great internal struggle within Harry as he has to choose between the man who raised him and what he believes is right for the wizarding community as a whole. There could be a lot of drama as Harry tries to find his place and figure out his true identity in this new and rather dark scenario.
Well, one possibility could be that Voldemort adopts Harry as a way to control the wizarding world. He might think that by having Harry on his side, the resistance against him will crumble. Another could be that Voldemort discovers some sort of connection between him and Harry that makes him want to take Harry under his wing, perhaps related to some long - forgotten prophecy.
One possible plot could be that Aunt Marge, surprisingly, has a change of heart. She starts to see Harry's potential and decides to take him in. She might try to 'muggle - ize' Harry, making him fit into her more ordinary world. This could lead to many comical situations as Harry tries to adapt to her rules while still having his wizarding side peek through now and then.
One possible plot could be that Snape, out of a sense of guilt or a newfound understanding of Harry's situation, decides to take him in. Snape would have to deal with Harry's grief over his parents and his own complex feelings towards the boy. He might start to teach Harry magic in a more personal and different way than at Hogwarts, leading to a unique bond between them.
Maybe it could be about the adventures they have in the orphanage. They might discover some secrets within the orphanage walls.
Perhaps in this fanfic, Tom and Harry discover a dark secret that shakes them to their core. This discovery makes Tom, who has his own complex character, unable to hold back his emotions and he cries along with Harry. It could be a secret about their families or the true nature of magic that they were not aware of before.
One popular 'Harry adopts a Tom fanfic' is 'A New Beginning'. In this fic, Harry discovers a young Tom in a difficult situation and decides to take him in. It explores how their relationship develops over time, with Harry trying to guide Tom away from the dark path he was destined for in the original story.
In a 'Percy Jackson adopts Harry Potter' fanfic, the plot could start with Percy being on some kind of mission that crosses into the wizarding world. He stumbles upon Harry, who is alone and vulnerable. Percy decides to adopt Harry because he sees the same kind of determination and courage in Harry that he has. As they journey together, they exchange knowledge. Percy teaches Harry about Greek mythology and combat skills, while Harry shares his knowledge of magic. They form a bond that helps them face threats from both magical and mythological creatures, and they might even team up to fight against a common enemy that is a threat to both their worlds.
Perhaps in the fanfic, Narcissa adopts Harry because she has a change of heart after seeing the destruction Voldemort causes. She wants to protect Harry as a way to atone for her family's association with the Dark Lord. Harry would then grow up in a Slytherin - influenced environment but still hold onto his Gryffindor values. This would lead to interesting interactions with other Slytherins and a different dynamic within Hogwarts.